Tungsten Inert Gas (TIG) Welding is widely applied in the fabrication of steels of various compositions and grades, and specially in welding of austenitic stainless steel components, in various industry sectors including Nuclear industry, Ship building, Chemical, Petrochemical, Power, Transportation, Automobile and their ancillary industries, in manufacturing as well as in maintenance jobs. The TIG welding process is considered as a reliable process for welding of a large variety of metals and alloys to ensure superior and almost defect free weld quality as the weld pool and the arc remain under the cover of inert envelop of argon gas and preferably applied as root pass for butt welded joint for difficult to access weld locations, where repair would be very difficult and costly. The TIG process is applied with or without the filler wire for a particular base metal composition, standard sections or cast or forged items. TIG welding is exclusively applied for welding of austenitic stainless steels to achieve defect free weld without degradation of mechanical properties and desirable microstructure.
The conventional TIG welding process, when applied for autogenous (i.e. without the addition of any filler material) weld bead of only 3 mm is achievable in austenitic stainless steels, in absence of use of any penetration activating flux. Some variation in weld penetration is found in such conventional autogenous TIG welding applications due to the variations in the composition of the base metal from one heat to another heat. Control on these variations and the enhanced penetration can only be effected by the use of activating flux of suitable composition deciding the desired end results of a TIG welded Joints, in terms of strength, microstructure, homogeneity and reliability in service. Hence there has been a need in the art to develop a flux for autogenous TIG welding specially of austenitic stainless steels that would favour controlling the afore discussed limitations and complexities of conventionally followed TIG welding applications and especially to improve upon the limit of penetration in the existing practice to further improve upon the productivity without affecting the quality of the weld.
U.S. Pat. No. 5,804,792 discloses an easy to apply flux for increasing the penetration of gas tungsten arc welding of stainless steel substantially independent of flux thickness and variations in composition from heat to heat of stainless steel includes a flux consisting of reagent or laboratory grade TiO or TiO.sub.2 (about 50%), Cr.sub.2 O.sub.3 (about 40%), and SiO.sub.2 (about 10%) in a liquid carrier, preferably of methyl ethyl ketone. The flux is stated to be easy to apply, increases penetration of the weld, decreases bead width, and increases weld cross sectional area. This Patent, however, is basically on the use of GTAW flux having enhanced penetration upto 6 mm in AISI type 304 stainless steel.
U.S. Pat. No. 6,664,508 discloses Deep penetration gas tungsten arc welds are achieved using titanates such as Na.sub.2 Ti.sub.3 O.sub.7 or K.sub.2 TiO.sub.3. A small amount of the titanate is applied to the weld zone in a carrier fluid paste or as part of a wire filler to afford deep penetration welds in carbon, chrome-molybdenum, and stainless steels as well as nickel-based alloys. To control arc wander, bead consistency, and slag and surface appearance of the weldments, various additional components may be optionally added to the titanate flux including transition metal oxides such as TiO, TiO.sub.2, Cr.sub.2 O.sub.3, and Fe.sub.2 O.sub.3, silicon dioxide, manganese silicides, fluorides and chlorides. However, the toxicity of the plumes from halide additives and the necessity of post-weld removal to prevent corrosion may preclude their use. In addition, it was found that a flux of titanium oxides, Fe.sub.2 O.sub.3 and Cr.sub.2 O.sub.3 affords deep weld penetration in carbon steels and nickel-based alloys but with some heat-to-heat variation.
U.S. Pat. No. 6,707,005 is directed to deep penetration gas tungsten arc welds are achieved using weld penetration containing one or more compounds selected from the group of compounds consisting of (a) a titanium oxide, (b) nickel oxide, (c) a metal silicide, and (d) mixtures of these compounds a flux containing at least two titanium oxides, nickel oxide, and a manganese silicide is particularly useful for welding a wide variety of material including nickel-based alloys and carbon and stainless steels. The flux can be applied as a paste, as part of a cored wire or rod coated on the exterior of a filler wire or rod. Alternatively, it can be mixed with a polymeric binder and applied to the weld zone as a hot melt, paint, tape, adhesive, rod, wire or a stick.
The above discussed prior U.S. Pat. Nos. 6,664,508 and 6,707,005 are found to claim use of penetration flux having enhanced penetration upto 10 mm in nickel base alloys and steels. While, the flux is attended to work also in case of austenitic stainless steel, the maximum penetration achieved was limited to 10 mm. However none of the above cited patent documents in related area, disclosed prior use of autogenous TIG process in single pass weld to improve welding productivity for such application, in addition to the objective of enhancement of weld penetration.
Thus there remains a continuing requirement in the art of TIG welding to achieve even greater penetration levels of welding involving penetration activating fluxes without affecting the quality of the weld to favour varied end use and applications and also to improve upon the productivity for welding by greater penetration of weld without degrading the microstructure and mechanical properties.